1. Field of the Invention
The present invention relates to a method and apparatus for use in the fabrication of heating elements, and more particularly to a method and apparatus for use in the fabrication of metal sheathed tubular electrical heating elements.
Electrical heating elements consisting of a coiled metallic wire fitted into a metallic tube and insulated from the tube by means of an insulating material are well known. They have a large number of applications, and are frequently used in immersion heaters, kettles, grills and the like.
2. Discussion of the Prior Art
There are several methods for manufacturing such elements, the most commonly used being the so-called “powder method”. In this method, the coiled electrical heating element is positioned in an upright metal tube. The tube is then filled with a refractory insulating powder (commonly magnesium oxide, MgO), which ensures that the heating element is insulated from the tube. The tube can be vibrated during filling to consolidate the powder. After filling, the tube is sealed, with pins at the ends of the heating element projecting from the tube to allow electrical connection to be made. The tube is then subjected to a rolling, swaging or similar operation to reduce its cross-sectional area and to compact the insulating powder, increasing its density. A density of 3 g/cm3 (3000 kg/m3) is typical for magnesium oxide. During the rolling or other process, the tube work-hardens, and so it is necessary to anneal the tube to allow it to be bent into the desired shape, in order for it to be assembled into the finished article.
It will be appreciated that this method includes a number of separate steps, and so can take some time. Further, the annealing step in particular involves high temperatures, and consumes a large amount of energy. It would be desirable to reduce the number of steps required to manufacture an element, and to omit the annealing step if possible.